In cooperation with five other U.S. Public Health Service Hospitals, a long-term prospective study is being undertaken to assess the effects of conventional antihypertensive drug therapy in patients with mild to moderate essential hypertension. At issue in this primary prevention study is the question of whether such treatment favorably influences the morbidity and mortality associated with the hypertensive state. An untreated group is being compared with one treated with a combination of a thiazide diuretic and a rauwolfia-serpentina preparation. Patients are selected who are free of discernible cardiovascular, renal, or cerebral complications. The incidence of these complications during follow-up will be monitored as specific end points.